The present invention relates to a method for correcting interpolation errors of a machine, such as but not limited to a coordinate measuring machine, and to a corresponding machine.
Coordinate measuring machines are typically used for determining individual spatial coordinates and/or the object shape of a test object by means of measuring technology, e.g. for quality control during the manufacture of workpieces. The known coordinate measuring machines comprise what is called a probe which is arranged on a displacement frame and can be moved in at least one spatial direction. There is often a stylus at the probe, which stylus is used to touch a selected measurement point on the test object. The spatial coordinates of the measurement point can then be determined from the position of the probe, wherein the deflections of the stylus upon touching are often also taken into consideration. Alternatively, there are also coordinate measuring machines which capture a measurement point on the test object without contact, e.g. using optical means.
In order to determine the position of the head in the measurement volume, coordinate measuring machines comprise material measures for the individual axes of travel. These material measures are often glass scales on which periodic divisions in the form of e.g. marking lines are applied. The periodic division is sampled by means of a sensor as the probe moves. The accuracy of the material measure therefore determines the measuring accuracy of the coordinate measuring machine.
In addition to glass scales, there also exist other material measures comprising a periodic division, e.g. material measures which are inductively sampled. Irrespective of the type of the material measure, each measurement of a coordinate measuring machine is prone to measuring errors which can have various causes. The causes include manufacturing tolerances and non-linearities in the guideways, deformations resulting from thermal influences and/or under load, manufacturing tolerances in the material measures and others. In order to reduce the effect of such measurement errors, it is known that the measured positional values which the coordinate measuring machine receives from its position measurement devices can be corrected mathematically. For this purpose, correction values are usually stored in the control and evaluation unit of the coordinate measuring machine, wherein the correction values themselves are determined from a reference measurement which is carried out using a laser interferometer, for example. Such a method is described in DE 1 638 032 A1, for example. This document also discloses performing an error correction by means of interpolation values, said interpolation values being determined from the interpolation of adjacent reference points.
Furthermore, interpolation values are often used in the case of coordinate measuring machines in order to mathematically increase the resolution and thus the measuring accuracy of the coordinate measuring machine. For example, DE 27 29 697 A1 describes a method for interpolating path-dependent and angle-dependent periodic signals of a photoelectric digital length-measurement or angle-measurement system. A general problem is here that the measuring accuracy ultimately depends on the quality of the interpolation values. Errors in the position signals which have been captured using measuring technology also adversely affect the interpolation values. DE 27 29 697 A1 therefore proposes that the measured values which are used for the interpolation be corrected before the interpolation algorithm is applied. These are typically digital values which are generated from the analog measurement signals of the material measure using A/D converters. It is proposed that the digital values be corrected in respect of symmetry, amplitude equality and 90° phase offset in order to eliminate corresponding fluctuations in the analog measurement signals.
A similar method is disclosed by DE 34 13 855 A1. This document also proposes the use of a signal curve which has been corrected in terms of amplitude, bias voltage and phase position as a basis for the interpolation calculation.
EP 0 048 851 B1 discloses the correction of interpolation values by means of correction values that were obtained from a reference measurement carried out using a laser interferometer. According to this, the interpolation values are calibrated in exactly the same way as the non-interpolated positional measured values of the coordinate measuring machine.
Furthermore, DE 34 26 863 A1 discloses the use of a rotatory (indirect) measurement system in addition to a linear material measure (designated therein as a direct measurement system), wherein the measurement resolution of the rotatory measurement system is approximately ten times higher than the measurement resolution of the linear position-measurement system. “Interpolation values” are thus obtained, i.e. by means of a second measurement system.
From DE 33 02 063 A1 it is known to superimpose correction values, which were determined for all possible positions, thermal states and load conditions, onto the measured actual values in a coordinate measuring machine, before the measured actual values are processed further. In order to avoid intervention in the control loop of the machine, the superimposition of the calculated correction values takes place in such a way that a corrected position value is simulated to the position control loop of the coordinate measuring machine. This is said to prevent a change in the stability characteristics.
A common feature of all known methods is that special hardware components are required for calculating and correcting the interpolation values, which is disadvantageous in terms of the manufacturing costs of the corresponding coordinate measuring machine.